The most important greenhouse gases currently accumulating in the atmosphere as a result of human activities are carbon dioxide, methane, nitrous oxide, sulphur hexafluoride and two groups of industrial gases known as hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs). These industrial gases are part of a longer list of greenhouse gases covered by the Kyoto Protocol which set commitments for developed countries to reduce greenhouse gas emissions for the period 2008 to 2012.
Chlorofluorocarbons (CFCs) and hydro-chlorofluorocarbons (HCFCs) are also powerful greenhouse gases, which were frequently used in refrigerators (among other uses). These two groups of gases are being progressively phased out under the Montreal Protocol as they also damage the stratospheric ozone layer, so they are not covered by the Kyoto Protocol.
The Global Warming Potential (GWP) of a greenhouse gas
Each greenhouse gas has a different capacity to cause global warming. To allow a comparison of the warming potential of each gas, an index known as the global warming potential (GWP) has been devised, which depends on the radiative properties and molecular weight of the gas, and how its atmospheric concentration decays over time.
The GWP of a gas is defined as its warming influence over a set time period relative to that of carbon dioxide, which by definition has a GWP of 1. For example, the GWP of the Hydro-fluorocarbon HFC-134 over a 100-year time horizon is 1,000. This means that 1 tonne of this gas emitted into the atmosphere has 1,000 times the warming potential over 100 years of 1 tonne of CO2. The time horizon is important because the GWP of a gas can have different values for different time horizons. The Kyoto Protocol sets a time horizon of 100 years in view of the relatively long time scale for addressing climate change.
When the warming effect of current greenhouse gas emissions over the next 100 years is calculated, the diagram below shows that carbon dioxide will be responsible for about two thirds of the expected future warming.
Relative warming effect of current emissions of greenhouse gases over next 100 years
Image: Pie Chart for CO2, CH4, N2O and others
Source: the Met Office's Hadley Centre for Climate Prediction and Research
To work out the carbon dioxide equivalent (CO2 eq) of the emission of any gas, we multiply its emission by its GWP. This is often expressed as the carbon equivalent, so we multiply GWP by 12/44 – the ratio of the atomic weights of carbon and carbon dioxide. So for example, an emission of 1 tonne of HFC-134 is equivalent to 1 x 1,000 = 1,000 tonnes of CO2 eq, or 1,000 x 12/44 = 273 tonnes of carbon.
Greenhouse gas emissions inventory and a list of Greenhouse Gases
The Greenhouse Gas emissions inventory and a list of Greenhouse Gases is available in the Statistics section of the DECC website